Lens for projection and photography.



c. at H. c. BECK. J LENS FOR PROJECTION AND PHOTOGRAPHY. APPLIOATIOIFILED Jul]; 6, 1911.

1,035,408. Patented Aug. 13, 1912.-

UNITED STATES PATENT OFFICE.

CONRAD BECK AND HORACE C. BECK, OF LONDON, ENGLAND.

LENS FOR PROJECTION AND PHOTOGRAPHY.

To all whom it may concern:

Be it known that we, CONRAD BECK and HORACE COURTHOPE BECK, subjects ofthe King of Great Britain, residing at London, England, have inventednew and useful Improvements in Lenses for Projection and Photography, ofwhich the following is a specification.

Our invention relates to combination lenses having three separatedlenses, two positive, one on each side of a negative.

The object of this invention is to construct an improved lens of thistype having a very large aperture preferably of at least one quarter itsfocus, in which the chromatic, spherical, astigmatic and other obliqueerrors are corrected to a great degree of accuracy over an angle of viewof about 30 to 40 degrees and in which the component lenses are notpowerful.

The two positive lenses are plano convex or approximately plano-convexand constructed of a glass with high refraction and low dispersion. Thenegative lens is an unequal biconcave lens of approximately the samerefractive index but greater dispersion. The radii of curvature arepreferably in the rat-i0 of 25 to 26. The radius of curvature of both ofthe positive lenses is approximately three tenths of the focal length ofthe finished lens, and the distances between the six surfaces of thecomplete lens may be as follows :Surface R to R 26 thousandths of thefocus of complete lens, surface R to R 100 thousandths of the focus ofcomplete lens, surface R to R 31 thousandths of the focus of com )letelens, surface R to R 62 thousandths o the focus of complete lens,surface R to R 26 thousandths of the focusof complete lens. These datamay be varied as regards curvature to a small extent, and as regardsdistances between the surfaces to a considerable extent. By this meanswe can construct a lens of a very large aperture not less than 2/9 ofthe focal length of the combination, and with curves of relatively largeradius of curvature, the individual lenses having relative long focallengths, all of them being more than 3/10 of the combined focus when thepositive power of the complete lens is equally distributed between thetwo positive components, but in any case the focal length of thenegative lens is more than 3/10 of the combined focus. Thus the cost ofa Specification of Letters Patent.

Application filed June 6, 1911.

Patented Aug. 13, 1912. Serial No. 631,534.

large aperture project-ion or photographic lens is very greatly reduced,

In the construction of corrected lenses the errors of a single positivelens are counteracted by similar but opposite errors in a negative lens.The combination of two such lenses of great power produces a lens of lowpower. In anastigmatic lenses the powers of the individual lenses havebeen great and the errors which are thus counterbalanced very great.Thus if as in our invention a formula can be devised in which thecomponent lenses need not be powerful the individual errors to becorrected are smaller and any slight alteration due to manufacture isless appreciable than when powerful lenses with great errors are used.Thus lower power individual lenses are much to be preferred. Low powerlenses;

are much more economical to manufacture as the curvature of the surfacesis less. This formula in our invent-ion for lenses of the large apertureof f4 or larger requires that the three lenses should have a refractiveindex approximately equal. We have found in our experience that theconditions required to correct radial and tangential astigmatism at thesame time as central aberration are more easily satisfied with a smallnumber of curves when the refraction of the glass is the same, adifference in refractive index requiring either a greater number ofcurves or more powerful component lenses. We find that there are certainadvantages in very small differences in the refraction of the glass forperfecting some of the other corrections which may be introduced withoutessentially altering our formula but if large alteration in therefractive indices exceeding about two per cent. be made our formulawould not apply. We have found that in order to make a lens that isequally serviceable for hotographing objects full size and for re ucingit is advantageous to have a system which is approximately symmetricalso that the curves are similar on either side of the center of thesystem, but we place our central lens slightly out of the center of thesystem and consequently may have slightly different curves on the twosides of the central lens though a difference greater than ten per cent.will damage the astigmatic corrections. The lens is also shorter inlength than is usual with large aperture lenses.

Refractive Dispcrs'ive Radii of curvature. index. ratw.

an uvalue Lens L R +3.392 inches 1.612 58.3

R 00 Lens L R 4.27 inches 1.618 37.7

R 4.11 inches Lens L R 49.5 inches 1.612 58.3

R 3.32 inches It will be noticed that the negative lens is bi-concaveand that one curve is more powerful than the other to the maximum extentof ten per cent.

Central distance of R to R .31 inch, central distance of R to R 1.15inch, central distance of R to R .31 inch, central distance of R to R.65 inch, central distance of R to R .31 inch; aperture of lens L 2.8inches, aperture of lens L .6 inches, aperture of lens L 2.8 inches.

The lens is used for photography with lens L nearest to the View, forprojection with lens L nearest to the enlarged image.

When the lens is to be made with an aperture notless than two ninths ofthe focal length of the combination the refractive index for the line Dof the glasses of the three lenses should not differ by more than twoper cent.

e do not limit ourselves to the refractive index of the glass used,considerable modification being possible with compensating alterationsin the separation between the lenses.

l/Vhat we claim is 1. A lens for projection and photog raphy,spherically, chromatically, and astigmatically corrected, consisting oftwo positive lenses and one negative lens separated by air spaces, inwhich the negative lens has a focal length greater than three tenths ofthe focal length of the combination.

2. A lens for projection and photography, spherically, chromatically,and astigmatically corrected, consisting of two positive lenses and onenegative lens separated by air spaces, with an aperture not less thantwo ninths of the focal length of the combination, in which the negativelens has a focal length greater than three tenths of the combination.

3. A lens for projection and photography, spherically, chromatically,and astigmatically corrected, consisting of two positive lenses and onenegative lens separated by air spaces, with an aperture not less thantwo ninths of the focal length of the combination, in which the negativelens has a focal length greater than three tenths of the focal length ofthe combination, and in which the refractive index for the line D of theglasses of the three lenses does not differ by more than two per cent.

at. A lens for projection and photography, spherically, chromatically,and astigmatically corrected, consisting of two positive lenses and onenegative lens separated by air spaces, with an aperture not less thantwo ninths of the focal length of the combination, in which the negativelens has a focal length greater than three tenths of the focal length ofthe combination, the

two positive lenses being lenses in which one curve has a power at leastsix times as powerful as the other, the negative lens being doubleconcave with one curve not more than ten per cent. more powerful thanthe other, the two positive lenses being placed on either side of thenegative lens.

5. A lens for projection and photography, spherically, chromatically,and astigmatically corrected, consisting of two positive lenses and onenegative lens separated by air spaces, with an aperture not less thantwo ninths of the focal length of the combination, in which the negativelens has a focal length greater than three tenths of the focal length ofthe combination, the two positive lenses being lenses in which one curvehas a power at least six times as powerful as the other, the negativelens being double concave with one curve not more than ten per cent.more powerful than the other, the two positive lenses being placed oneither side of the negative lens, and in which the refractive index forthe line D of the glasses of the three lenses does not differ by morethan two percent.

In witness whereof we have hereunto set our hands in presence of twowitnesses.

CONRAD BECK. HORACE O. BECK.

/ Witnesses:

ALFRED T. PATTEN, WM. GIRLING.

Copies of this patent may be obtained for five cents each, by addressingthe Commissioner of Patents. Washington, D. G.

